1. Field of the Invention
The present invention relates to a backlight unit for use in a liquid crystal display device, and more particularly, to a direct-irradiation type backlight unit in which a light source is disposed to oppose a light-emitting surface of the backlight unit.
The present invention also relates to a liquid crystal display device having such a backlight unit.
2. Description of Related Art
A backlight unit for use in a liquid crystal display (LCD) device is roughly classified into two types. One is a direct-irradiation type and the other an edge-light type having an optical guide plate. In the direct-irradiation type backlight unit, a plurality of light sources (lamps) are disposed to oppose a light-emitting surface of the backlight unit. A higher luminance can be easily obtained in the direct-irradiation type backlight unit as compared to the edge-light type backlight unit.
FIG. 10 is an exploded view showing the structure of an LCD device generally designated by numeral 200. A liquid crystal (LC) panel 202 of the LCD device 200 is sandwiched and held between a shield frame 203 and a backlight unit 201, and controls a transmission of the light emitted from the backlight unit 201 to thereby display images on the screen of the LC panel. FIG. 11 is an exploded view showing in detail the structure of the backlight unit shown in FIG. 10. FIG. 12 includes a sectional view showing a part of the backlight unit 201 taken along the line A—A of FIG. 10, and a graph showing the luminance distribution on a diffusion plate 216 of the backlight unit shown in the sectional view.
As shown in FIG. 11, the backlight unit 201 includes a backlight chassis 218 for defining the light-emitting surface of the backlight unit 201. Lamps 213 are disposed between a reflection plate 211 and the diffusion plate 216, and supported by a pair of lamp supporting members 215 in such a manner that the lamps 213 are spaced apart from each other at a predetermined interval. For example, the distance between the diffusion plate 216 and the center of the lamp 213 is set at about 10 to 17 mm; and the distance between each adjacent two of the lamps 213 is set at about 20 mm. One input terminal of the lamps 213 is grounded through a return cable 214. To the other input terminal is applied, by an inverter 219, a high alternating voltage of about 1000 to 1600 V at a peak, which is generally called a lighting start voltage. A larger length of the lamp 213 increases the lighting start voltage.
When the lamp 213 is disposed in an environment such as a dark state or low temperature state, the impedance of the lamp 213 is increased. It becomes, therefore, necessary to set the lighting start voltage higher than usual, assuming that the length of the lamp 213 is not changed. This leads to an increase in the size and cost of a power supply board, or inverter 219, for supplying the electric power to the lamp 213. When a conductive material is disposed in the vicinity of the lamp 213, an electrical discharge in the lamp 213 is induced by a leakage current 220 flowing between the lamp 213 and the conductive material. Thus, it becomes possible to lower the lighting start voltage even in the case of a dark state or low temperature state. For this reason, in the backlight unit 201, a metal is often used for the reflection plate 211.
The diffusion plate 216 has opposing edges supported by the lamp supporting member 215 and a central portion supported by a spacer 212 which is attached onto the reflection plate 211. The light emitted from the lamp 213 enters the diffusion plate 216 directly or by reflection from the reflection plate 211. The light incident onto the diffusion plate 216 then diffuses through an optical sheet assembly 217 such as including a diffusion sheet and a lens sheet, and exits the backlight unit 201.
In the backlight unit 201, the distance between the lamp 213 and the diffusion plate 216, and the angle of the light incident onto the diffusion plate 216 generally change depending on the position as viewed in the X-direction perpendicular to the extending direction of the lamp 213 in FIG. 11. Therefore, on the surface of the diffusion plate 216, the highest luminance is observed at the position right above the lamp 213, and the lowest luminance is observed at the position right above the intermediate position between each adjacent two of the lamps 213, as will be understood from the graph of FIG. 12. In this case, there arise a problem in that a luminance irregularity is observed on the light-emitting surface of the backlight unit 201. The backlight unit 201 also has the problem that the light, which is emitted from a lamp 213, travels in X-direction and illuminates the other two lamps 213 disposed adjacent to the former, are absorbed by the surfaces of the latter, resulting in a low luminance efficiency.
Patent Publications JP-A-4-275525 and -10-39808 describe an LCD device that can solve the above problems. FIG. 13 shows a sectional view of the backlight unit described in JP-A-4-275525, wherein the reflection plate 211 includes a convex portion 211M having a peak at the intermediate position between the adjacent two lamps 213. A reflection film having a mirror surface is formed on the convex portion 211M. With this configuration, the light emitted from the lamp 213 and traveling in the X-direction is reflected by the mirror surface of the reflection film formed on the convex portion 211M of the reflection plate and then travels in the direction toward the diffusion plate 216. This reduces the luminance irregularity observed on the light-emitting surface and thereby increases the luminance efficiency.
A double-sided display LCD device is known such as described, for example, in Patent Publication JP-A-2000-338483. The double-sided display LCD device has a display screen on the front side as well as on the rear side of the backlight unit. FIG. 14 is an exploded view showing the structure of the double-sided display LCD device. As shown in FIG. 14, the doublesided display LCD device 200a includes a liquid crystal (LC) panel 202 and a shield frame 203 on the front side (shown at the top in the drawing) as well as on the rear side (shown at the bottom in the drawing) of a double-sided backlight unit 204. The double-sided backlight unit 204 has a structure such as obtained by bonding the rear sides of two of the single-sided backlight units .
FIG. 15 is an exploded view showing the structure of the double-sided backlight unit described in JP-A-2000-338483. FIG. 16 is a sectional view showing a part of the backlight unit 204 and taken along the line B—B of FIG. 14. In this double-sided backlight unit 204, the diffusion plates 216 are disposed near both the front and rear sides of the backlight unit 204 to sandwich therebetween the lamps 213. Unlike the single-sided backlight unit shown in FIG. 12, the double-sided backlight unit 204 does not include the reflection plate 211 opposing the diffusion plate 216, allowing double-sided emission by a plurality of the lamps 213 arranged in a line. As shown in FIG. 16, which shows the double-sided backlight unit 204 similarly to FIG. 12, the double-sided backlight unit 204 has a symmetric structure with respect to the center of the lamps 213 disposed at the center of the backlight unit.
As will be understood from the graph of FIG. 16, also in the double-sided backlight unit 204, the luminance efficiency is lower and the undesirable luminance irregularity is observed on the front- and rear-side diffusion plates 216 similarly to the single-sided backlight unit 201 having the cross section shown in FIG. 12. Further, as described above, the double-sided backlight unit 204 does not include the reflection plate unlike the single-sided backlight unit 201 that includes the reflection plate 211 disposed on the rear side of the lamp 21. Therefore, the technique used in JP-A-4-275525 cannot be applied to the double-sided backlight unit 204 for solving the above problems. Further, in the double-sided backlight unit, the absence of the reflection plate 211 made of a metal makes it impossible to lower the impedance of the lamp 213 and thereby to raise the lighting start voltage especially in the case of a dark state or low temperature state.